Speckles are usually observed when a coherent light probe beam is scattered from a non-uniform surface or material. A typical coherent light probe beam is a laser beam. When such a coherent light probe beam hits a non-uniform surface or material, scattered lights from different scatters interfere with each other to form a speckle pattern in an observation screen.
Speckles limit the image quality of coherent light probe beam on a surface or material. In many applications, image quality of the probe beam on a surface or material can be important. For example, in an eye diagnosis instrument called wavefront analyzer, a probe beam is directed into a subject eye and intersected with the retina. The probe beam in this application needs to be narrow and near collimated such that the beam spot size is small on both the cornea and the retina. Scattered light from the retina exists from the pupil and is then collected to form, through a lenslet array called Hartmann-Shack sensor, a multiple-spots image of the probe beam on a CCD camera. In such an instrument, the centroid of each spot image shall be accurately determined in order to calculate the eye's aberration. However, speckles distort the quality of the spot image and degrade the precision of the centroid measurement.
In many applications, using a laser as a probe beam can be preferable while speckles are troublesome. A laser may be chosen as a probe beam for its superior beam quality, its incomparable brightness, its narrow bandwidth, and/or its wavelength availability. However, as a good coherent light source, laser probe beam produces most significant speckles. In the application of wavefront analyzer, for instance, a laser beam should be an ideal probe beam if there were no speckles.
A super luminescence diode (SLD) may replace a laser for a probe beam when high brightness, good beam quality, but less speckle is required. SLD probe beam has shorter coherent length than a laser beam and produces weak speckles when particularly the scatters are located at different depth along the beam. However, spatial coherence across the beam from a SLD is substantial the same as that from a laser. Therefore, speckle reduction with a SLD is not complete, especially for scattering from scatters on a surface. Besides, SLD has much limited selections in term of power, wavelengths, and vendors.
It is well known in the art that speckles can be diminished via either moving the coherent light probe beam or moving the scattering surface. In U.S. Pat. No. 6,199,986 B1 to Williams et al., a SLD probe beam is used in an experimental setup to measure eye's wave aberration and a turning mirror scans the probe beam rapidly to eliminate speckles in the Hartmann-Shack image, i.e. the multiple-spot image of the probe beam on retina. However, moving the probe beam in an eye diagnosis instrument is difficult to implement without effecting the measurement accuracy and reliability.
It is also well known in the art that speckles can be eliminated with a moving diffuser placing in the beam path of a coherent light beam. In U.S. Pat. No. 5,851,740 to Sawyer, a moving diffuser is used to reduce the speckles in a setup in which laser light is used as an illumination light source. In general, a diffuser diffuses light in a wide angle and degrades significantly the beam quality of a laser beam. Obviously, beam quality is not a design issue to consider when the laser beam is used only as an illumination light source, such as in the application described in U.S. Pat. No. 5,851,740.